The present invention generally relates to a dielectric resonator including at least one dielectric column disposed within a cavity of a casing formed with an electrically conductive film over its surface.
More particularly, the present invention relates to a band elimination filter which employs a dielectric resonator including a dielectric column means provided within a ceramic casing formed with an electrically conductive film over its inner or outer surface, and also, to a dielectric filter including at least one dielectric column disposed in a ceramic casing formed with a ground electrode generally over its entire inner or outer surface.
Conventionally, band elimination filters are generally constructed from dielectric coaxial resonators or coaxial cables.
In the known band elimination filter of the above described type, however, there has been the problem that, since the quality factor Q is low, for example, in the vicinity of about 1500 to 2000; when the frequency is 800 MHZ, a sufficiently large attenuation can not be achieved, particularly in the case of a band elimination filter with a small number of stages.
Although the problem as described above is considered to be solved by a TM-mode (transverse magnetic mode) dielectric resonator having a higher quality factor Q, it has been difficult in actual practice to provide a desired band elimination filter based on such a dielectric resonator. More specifically, in order to constitute a band elimination filter, a resonator having a reactance connected in parallel therewith must be connected to a .lambda./4 line in a branched arrangement. However, since the TM-mode dielectric resonator as referred to above has a closed construction in which the dielectric column is provided within the cavity of a ceramic case which has an the electrically conductive film formed over its inner or outer surface, with opposite ends of the dielectric column adapted to contact said casing, there is still the disadvantage that connection thereof with an external circuit or the like is impossible unless the hard and brittle ceramic casing is subjected to drilling for the formation of holes.
FIG. 18 shows another example of a conventional band-pass filter including six dielectric columns 2 which serve as TM-mode dielectric filters. The respective dielectric columns 2 are disposed at predetermined intervals in a ceramic casing 1 in which its inner cavity is divided by a partition wall 5 Generally over an entire outer surface or inner surface of the ceramic casing 1, a ground electrode (not shown) which serves to define a cavity is formed, and each this ground electrode and one of the dielectric columns 2 equivalently form one resonator. By disposing the respective dielectric columns 2 at predetermined intervals within the cavity, the neighboring resonators are electro-magnetically coupled to each other. Accordingly, the dielectric filter shown in FIG. 18 constitutes a bandpass filter in which six resonators are coupled with each other.
For coupling the above dielectric filter with external circuits, coaxial connectors 3 and 4 which are connected with the external circuits through cables (not shown) are mounted on one side wall of the ceramic casing 1 as shown in FIG. 18. Loop antennas or the like (not particularly shown) are inserted into the cavity in the casing 1 from the respective connectors 3 and 4, whereby the resonators at the input and output stages and the external circuits are coupled through said antennas.
It is to be noted that the casing 1 is formed of a ceramic material in order to improve the temperature characteristics of the dielectric filter by making the coefficient of linear expansion of the casing 1 equal to that of the dielectric columns 2 as far as possible.
In connection with the above, a problem with the dielectric filter in which the ceramic material is employed for the casing 1, is that since the ceramic material is hard and brittle, it is very difficult to form holes for attaching the input and output connectors 3 and 4 on said casing 1.
On the other hand, it was proposed, for example, in Japanese Patent Application Tokugansho 60-298149 assigned to the same assignee as the present invention, to form poles or a pole at both sides or one side of a band-pass region for obtaining a desired band-pass region (referred to as polarization hereinafter). However, there is the problem that, although the polarization is desired in the dielectric filter as described above, realization thereof is very difficult. More specifically, in order to provide the polarization, resonators disposed at opposite sides of at least one resonator, positioned therebetween, must be coupled with each other by a reactance element, but in the dielectric filter as described so far, since all of the dielectric ,columns are sealed in the ceramic casing, it is impossible to couple two of the dielectric columns by such a reactance element without subjecting the ceramic casing to the difficult drilling for forming holes.